Multiresolution subdivide modifier on a mirrored surface causes a visible seam
First I presumed that you did mirroring by negative scale around YZ-plane, then you have flipped normals at second half. But then I noticed modifier, so I was wrong.

I noticed similar result if Subdivision modifier placed higher than Mirror. Also Blender prevents from moving Multiresolution with subdivisions under Mirror, saying "Cannot move beyond non-deforming modifier" or "Cannot move above modifier requiring original data" for moving Mirror up. 

I think it's a way Blender modifiers works with mesh data. I took a peek in official docs and found this: https://www.blender.org/manual/modifiers/the_stack.html - according to official info modifiers applied in order from highest to lowest. When I tried to apply them in this order, I also get seam in final mesh with both modifiers, but it was just crease in geometry, vertices are wielded, normals are outside, just little wrong geometry. Old edge at section at half-mesh left unsmoothed by modifier, because it processed differently areas where edges belong to just one face.